1. Field of the Invention
The present invention pertains to a method and apparatus for addressing liquid crystal devices. More particularly the present invention pertains to a method and apparatus for addressing high information content, direct multiplexed, rms responding liquid crystal displays.
2. Discussion of the Prior Art
Examples of high information content direct multiplexed, rms-responding liquid crystal displays are systems that incorporate twisted nematic (TN), supertwisted nematic (STN), or superhomeotropic (SH) liquid crystal display (LCD) panels. In such panels, a nematic liquid crystal material is disposed between parallel-spaced, opposing glass plates or substrates. In one common embodiment, a matrix of transparent electrodes is applied to the inner surface of each plate, typically arranged in horizontal rows on one plate and vertical columns on the other plate to provide a picture element or "pixel" wherever a row electrode overlaps a column electrode.
High information content displays, such as those used in computer monitors, require large numbers of pixels to portray arbitrary information patterns in the form of text or graphic images. Matrix LCDs having 480 rows and 640 columns forming 307,200 pixels are commonplace, although it is expected that matrix LCDs may soon comprise several million pixels.
The optical state of a pixel, e.g. whether it will appear dark, bright or an intermediate shade, is determined by the orientation of the liquid crystal director within that pixel. In so-called rms responding displays, the direction of orientation can be changed by the application of an electric field across the pixel which field induces a dielectric torque on the director that is proportional to the square of the applied electric field. The applied electric field can be either a dc field or an ac field, and because of the square dependence, the sign of the torque does not change when the electric field changes sign. In the direct multiplexed addressing techniques typically used with matrix LCDs, the pixel sees an ac field which is proportional to the difference in voltages applied to the electrodes on the opposite sides of the pixel. Signals of appropriate frequency, phase and amplitude, determined by the information to be displayed, are applied to the row and column electrodes creating an ac electric field across each pixel which field places it in an optical state representative of the information to be displayed.
Liquid crystal panels have an inherent time constant .tau. which characterizes the time required for the liquid crystal director to return to its equilibrium state after it has been displaced away from it by an external torque. The time constant .tau. is defined by .tau.=.eta.d.sup.2 /K, where .eta. is an average viscosity of the liquid crystal, d is the cell gap spacing or pitch length and K is an average elastic constant of the liquid crystal. For a conventional liquid crystal material in a 7-10 .mu.m cell gap, typical for displays, the time constant .tau. is on the order of 200-400 ms.
If the time constant .tau. is long compared to the longest period of the ac voltage applied across the pixel, then the liquid crystal director is unable to respond to the instantaneous dielectric torques applied to it, and can respond only to a time-averaged torque. Since the instantaneous torque is proportional to the square of the electric field, the time-averaged torque is proportional to the time average of the electric field squared. Under these conditions the optical state of the pixel is determined by the root-mean-square or rms value of the applied voltage. This is the case in typical multiplexed displays where the liquid crystal panel time constant .tau. is 200-400 ms and the information is refreshed at a 60 Hz rate, corresponding to a frame period of 1/60 s or 16.7 ms.
One of the main disadvantages of conventional direct multiplex addressing schemes for high information content LCDs arises when the liquid crystal panel has a time constant approaching that of the frame period. (The frame period is approximately 16.7 ms). Recent technological improvements have decreased liquid crystal panel time constants (.tau.) from approximately 200-400 ms to below 50 ms by making the gap (d) between the substrates thinner and by the synthesis of liquid crystal material which has lower viscosities (.eta.) and higher elastic constants (K). If it is attempted to use conventional addressing methods for high information content displays with these faster-responding liquid crystal panels, display brightness and contrast ratio are degraded and in the case of SH displays, alignment instabilities are also introduced.
The decrease in display brightness and contrast ratio occurs in these faster panels because with conventional multiplexing schemes for high information content LCDs, each pixel is subjected to a short duration "selection" pulse that occurs once per frame period and has a peak amplitude that is typically 7-13 times higher than the rms voltage averaged over the frame period. Because of the shorter time constant .tau., the liquid crystal director instantaneously responds to this high-amplitude selection pulse resulting in a transient change in the pixel brightness, before returning to a quiescent state corresponding to the much lower ms voltage over the remainder of the frame period. Because the human eye tends to average out the brightness transients to a perceived level, the bright state appears darker and the dark state appears brighter. The degradation is referred to as "frame response". As the difference between a bright state and a dark state is reduced, the contrast ratio, the ratio of the transmitted luminance of a bright state to the transmitted luminance of a dark state, is also reduced.
Several approaches have been attempted to reduce frame response. Decreasing the frame period is one approach, but this approach is restricted by the upper frequency limit of the driver circuitry and the filtering effects on the drive waveforms caused by the electrode sheet resistance and the liquid crystal capacitance. Another approach is to decrease the relative amplitude of the selection pulse, i.e., decreasing the bias ratio, but this ultimately reduces the contrast ratio.
Other matrix addressing techniques are known which do not employ high-amplitude row selection pulses and therefore would not be expected to induce frame response in faster-responding panels. However, these techniques are applicable only to low information content LCDs where either there are just a few matrix rows or where the possible information patterns are somehow restricted, such as in allowing only one "off" pixel per column.
One advantage of the faster responding liquid crystal panels is that it makes video rate, high information content LCDs feasible for flat, "hang on the wall" TV screens. However, this advantage cannot be fully exploited with conventional direct multiplexing addressing schemes because of the degradation of brightness and contrast ratio and the introduction of alignment instabilities in these panels caused by frame response.